The present invention concerns a method for drawing iron and steel wire rod. More particularly, it concerns an improvement of the pretreatment method where wire drawing is conducted by using a dry lubricant.
The pretreatment process for iron and steel rod prior to wire drawing hitherto employed generally comprises descaling such as shot blasting, polishing, pickling etc. or phosphate conversion treatment after descaling/neutralizing/temporary rust preventive treatment/drying/lubrication treatment. This pretreatment process typically consists of degreasing and water rinsing of iron and steel rod if needed, pickling with dilute acid of HCl or H.sub.2 SO.sub.4 for descaling, water rinsing, neutralization, temporary rust prevention treatment, drying. After that, the wire rod is drawn by using powder lubricant. The drawn wire coated with rust preventive oil for the purpose of temporary rust protection is delivered to the user, where it is drawn into sewing-machine needles, springs, bicycle spokes, etc. which may then be cleaned, water rinsed, pickled and plated. As the abovementioned neutralizing/temporary rust preventive treatment solution, in general, "lime-soap" solution prepared by mixing/dissolving quick lime and needle-shaped soap into water is used while maintaining its temperature at 50.degree.-80.degree. C.
Another lubrication treatment is to use liquid lubricant in place of powder lubricant. As to such lubricant, there are metal soap solution type and mixed aqueous solution of sodium phosphate, borax and titanium oxide (Japanese patent publication Sho No. 30-2358), which are all publicly known. The metal soap solution type is accompanied by viscosity rise during use, often leading to the lack of uniform coating formation or insufficient drying, and eventually resulting in insufficient lubrication effect. For this reason, powder lubrication mentioned earlier is in common use.
Powder lubricant is, in general, formulated with metal soap of various kinds as the base and inorganic substances like lime, etc. with addition thereto of sulphur, MoS.sub.2, etc. Such powder lubricant is filled in a box in front of drawing die and used to pass iron and steel wire rod therethrough and to form lubricant film on steel surface.
The present invention attempts to solve the problems arising from pretreatment of iron and steel rod to be wire-drawn with use of a powder lubricant.
In the abovementioned case, if the drawing process is performed without sufficient lubricity, surface defects take place. If the drawn products are intended for, e.g., sewing-machine needle or others that require stringent surface finishing accuracy, the condition is to be with no defect under microscopic observation of 2000 magnification. Any defect or surface irregularity may lead to a rough surface of plated film and eventually result in impaired appearance of the final products. Therefore, in drawing iron and steel rods, the higher the cosmetic grade required for the final products, the more stringent becomes the need for the lubricant performance.
Lubricity of the powder lubricant is affected by the composition. Even in case of using a metal soap of high quality, sometimes so-called hair line defects may take place on drawn products during processing.
Further in case bar in coil form to be drawn is left standing for long time after having been pickled, neutralized and temporary rust prevention treated, it is often the case that rust develops and the neutralization/temporary rust preventive coating absorbs water. As a remedy, reprocessing with pickling-neutralization/temporary rust prevention is sometimes done. The problem still remains, however, that the "lime soap" film of neutralization/temporary rust prevention is hard to remove.
Surface conditioning chemicals for use prior to phosphate conversion treatment are publicly known (e.g. Japanese Patent Publications Nos. Sho 39-7125, Sho 58-55529 and Sho 60-41148: U.S. Pat. No. 2,310,239 and U.S. Pat. No. 2,874,081), wherein the basic constituent is colloidal titanium compound and alkaline phosphate. The fundamental function is to activate or condition the substrate metal surface for the formation of uniform, fine and dense phosphate coating. In the present invention, the exact function of the surface conditioning agent known for use prior to phosphate conversion treatment (herein after referred to as surface conditioner) to the intermediate process before metal soap treatment of wire rod and after pickling or phosphate treatment thereof remains unclear. Presumably, the film of surface conditioner existing between steel wire surface and metal soap film plays a role in retaining metal soap powder particles in a more favorable state. When wire is drawn and compared between the cases with and without use of the surface conditioner, the former shows requirement of reduced drawing power, with extremely reduced damage occurrence frequency; also such phenomena are observable that the colloidal titanium compound remains on wire surface even after drawing.